Microelectromechanical systems (MEMS) are small, generally movable devices which are made using semiconductor integrated circuit fabrication techniques. Because of these batch processing techniques, large numbers of small MEMS devices can be made on a single wafer substrate at low cost with high precision. MEMS devices typically have dimensions on the order of microns, and can thus be used to make very small actuators which are capable of very small and precise movements. Such actuators can make use of any of a number of phenomena to produce motion in the movable member. MEMS actuators are known which use electrostatic, thermal, magnetostatic and piezo electric effects, for example, to produce motion in the movable actuator member.
Microelectromechanical systems (MEMS) techniques may therefore be used to produce microfabricated piezoelectric actuators. Piezoelectric materials are those which undergo a strain when a voltage is applied, or generate a voltage when a stress is applied. Prior art actuators exist which use piezoelectric materials, and may be used as the pumping mechanism for a microfluidic pump. The piezoelectric microactuator can be made by depositing a stack of piezoelectric layers on a thin diaphragm which defines the pumping chamber. Application of a voltage to the piezoelectric stack results in a deformation of the diaphragm, which draws the fluid into the chamber through an inlet valve. When the voltage is discontinued, the diaphragm returns to its original shape, forcing fluid out of the chamber through an outlet valve. Piezoelectric microactuators generally produce a force perpendicular to the plane of the substrate on which they are deposited, and thus move primarily in this direction. A thorough analysis of the attributes of such a pump is set forth in “Simulation of MEMS Piezoelectric Micropump for Biomedical Applications”, which discusses the speed and displacement of such an actuator, and can be accessed at http://www.algor.com/news_pub/tech_white_papers/MEMS_micropump/default.asp.
Lead zirconate titanate, Pb(Zr,Ti)O3 (PZT), is a common piezoelectric material that can be deposited on silicon wafers by RF sputtering, for example. However, care must be taken to relieve the stresses in the deposited material in order to avoid static deformation, or warpage, of the pumping diaphragm. Alternatively, high performance PZT wafers are also under development; however they are not yet available in sufficiently large (150 mm round) format to facilitate wafer-to-wafer bonding, an essential process for low cost manufacturing. Accordingly, the exemplary piezoelectric micropump discussed above is an idealized case, with zero residual stress, and such pumps tend to be expensive and difficult to fabricate.
This technology has several other drawbacks, the most significant of which are that the piezoelectric actuator has limited throw and requires large actuation voltages. If non-resonant excitation of the above structure is used to actuate the diaphragm, the displacement of the design described above is less than 10 μm for a 200V input. If resonant excitation is used; i.e. a modulated voltage waveform is applied to the device to amplify the displacement, a ten fold increase in the displacement can be achieved; however, it takes about 100 msec to achieve this displacement. The low resonant frequency is a result of the weight of the piezoelectric material and the size of the pumping diaphragm needed to achieve the necessary pumping volume. The mass of the volume of fluid may also play a role in the low resonant frequency. If the pump is operated above this resonant frequency, the displacement is greatly diminished to only about 3 μm at 500 Hz for 200V input.
Furthermore, when used in a pumping device, the piezoelectric device described above has chambers and a layout that do not allow the passage of relatively large particles. For example, particles in excess of about 10 μm will not pass readily through the fluid path, because of the severe turns and small apertures in the path. Vertical pumps such as that described may also be relatively expensive and difficult to fabricate, because the valves are necessarily formed vertically below the diaphragm using other layers. Finally, since the piezoelectric material can only generate a strain in a single direction in response to an applied voltage, the actuator can only deform in one direction, i.e. it can only “push” and cannot “pull”.
Accordingly, a need exists for a microactuator capable of delivering small volumes of fluids as well as particulate matter suspended in the fluid stream, and which is inexpensive and easy to fabricate. The microactuator ideally operates at low voltages and is capable of being powered by a small battery.